1. Field of the Invention
This invention relates to a method and apparatus for pultruding fiber-reinforced, thermoplastic stock and, more specifically, it relates to such a system which is adapted to efficiently pultrude such stock made from thermoplastic resin film and provides means interposed between the array of dry reinforcing materials being formed and the die to maintain the desired surface quality of such reinforced thermoplastic resin stock.
2. Description of the Prior Art
The beneficial use of composite materials composed of a resinous material reinforced with a fiber has long been known. Such materials are frequently desirable in terms of their strength-to-weight and stiffness-to-weight characteristics, as well as corrosion resistance. They also have other desirable properties such as improved damage tolerance, reduced life cycle cost, and custom design flexibility through anisotropy.
Among the industries wherein such composite materials have found substantial use are the aerospace and transportation industries. While both thermosetting and thermoplastic resins are available for use in fiber reinforced composites, the majority of composites fabricated today employ thermosetting resins. These thermosetting materials generally require long processing cycles for the reaction kinetics to occur and in some cases require a lengthy post-cure. All of this results in increased part costs. Also, thermosetting materials have limited shelf life and require refrigeration and monitoring to ensure that the kinetics do not advance too far prior to fabrication of the product.
By contrast, thermoplastic polymers are limited in processing time solely by the rates of melting, consolidation and cooling. A major limitation to the use of thermoplastics in reinforced composites is their high melting viscosities which are in the order of about 10.sup.5 to 10.sup.6 cP as compared with 10.sup.2 to 10.sup.3 cP for thermosetting material. These higher thermoplastic viscosities tend to lead to complications in fiber wet-out and void removal which are critical properties. While it has been known to employ solvents to reduce thermoplastic polymer viscosities, in-process removal, collection, disposal and/or recycling add to the cost of thermoplastic manufacturing processes. Also, most solvents are considered potentially hazardous to the environment and to human health. In spite of these advantages of thermoplastics, there remains a substantial need to provide an economical means of effectively producing fiber-reinforced thermoplastic resin materials.
U.S. Pat. No. 4,164,439 discloses a method of making fiber-reinforced plastic grates out of reinforcing fiber and thermosetting plastic resins. This method involves the use of tooth containing belts around which the fibers are wrapped. A pair of adjacent belts orbit in opposite directions and are stationary for a two minute period during the molding operation.
U.S. Pat. No. 4,440,593 is directed toward creating by the pultrusion process reinforced plastic composite articles by the use of curved dies. In the embodiment illustrated in FIGS. 13 through 16 a belt-like die member is urged into cooperation with another belt member by a plurality of rollers to create a die chamber therebetween. See also the curved pultrusion system of U.S. Pat. No. 3,873,399 wherein reinforced plastic articles of arcuate shape are created. A pair of driven belts are employed as a means for pulling the composite through the downstream die elements.
U.S. Pat. No. 3,847,707 discloses a dual doctor blade system wherein the blades serve to coat and control the reinforcing fiber. Reference is made to predominant use of thermosetting resins. The exterior film portions are provided from opposite sides of the fibers and are urged into intimate contact to form the exterior plies of the product.
U.S. Pat. No. 4,886,701 discloses providing a blend of glass fibers and binder powder, compacting the same and then delivering it by conveyor means, which may be a belt, to a compacting station where heat may be provided to create an intermediate product. A press belt may be used.
U.S. Pat. No. 4,622,192 discloses a glass/carbon fiber composite using a polyolefin. It discloses the use of heated steel belts with the laminate being compressed by pressure rolls.
Copending U.S. patent application Ser. No. 593,185 owned by the assignee of the present application discloses the use of pultrusion to create a fiber-reinforced thermoplastic stock without having to draw the fiber-reinforcements through a resin bath prior to entry into the die. Means for maintaining a pool of thermoplastic are provided at or near the die entrance.
In spite of the foregoing disclosures there remains a real and substantial need for a method and apparatus for creating fiber-reinforced thermoplastic composites by the pultrusion method in a rapid and economical manner so as to permit thermoplastics to have greater use in fiber-reinforced composite products.